Various proposals for a protection circuit against Electrostatic Discharge (ESD) have been made heretofore. ESD means a discharge from an electrically charged body or machine to a semiconductor device and a discharge from an electrically charged semiconductor device to the ground potential. When an ESD occurs in a semiconductor device, a large amount of electric charge from the terminal flows into the semiconductor device as a current and the electric charge generates a high voltage inside the semiconductor device, hence causing, for example, a dielectric breakdown of the internal elements and subsequently a fault in the semiconductor device.
A representative example of the electrostatic protection circuit is an RC triggered (RCT) MOS circuit. This RCT MOS circuit uses a trigger circuit formed by a serial circuit including a resistor and a capacitor serially connected between power terminals. A voltage at the connection point between the resistor and the capacitor is set as a trigger signal for driving a shunt transistor for discharging the electrostatic surge. Since the on time of the shunt transistor is determined by a time constant of the trigger circuit, the time constant used in the trigger circuit has to be large enough to allow discharge of the ESD surge completely. However, when a large current keeps flowing through the shunt transistor for a long time, there is concern that the shunt transistor itself may overheat and be damaged.